The effectiveness of a new potent diuretic, xipamide, was evaluated in the treatment of patients with mild to moderate essential hypertension. The effects of daily doses of 40 mg xipamide were compared with those of 100 mg chlorthalidone with respect to systolic and diastolic blood pressure, using a double-blind crossover design. Patients received each drug for 6 weeks, the order of treatments being at random and the periods being separated by a 'washout' period of 7 days. The results showed that xipamide was as effective as chlorthalidone in controlling blood pressure, both in the upright and supine positions, and when administered after chlorthalidone produced a further reduction. No such further reduction occurred when chlorthalidone was given after xipamide. Serum electrolyte changes induced by the two diuretics were comparable. Both drugs were well tolerated and caused few side-effects.